aes-gcm-nonce-reuse.rb

require 'openssl'

def encrypt(plaintext)
  key = "\1" * 16
  nonce = "\2" * 12
  aes = OpenSSL::Cipher.new('aes-128-gcm')
  aes.encrypt
  aes.key = key
  aes.iv = nonce
  aes.auth_data = ''
  ciphertext = aes.update(plaintext) + aes.final
  tag = aes.auth_tag
  return [ciphertext, tag]
end

def xor_strings(str1, str2)
  min = str1.bytesize < str2.bytesize ? str1.bytesize : str2.bytesize
  bytes1 = str1.byteslice(0...min)
  bytes2 = str2.byteslice(0...min)
  bytes1.bytes.zip(bytes2.bytes).map { _1 ^ _2 }.pack('c*')
end

# Encrypt
plaintext1 = 'hello world!'
plaintext2 = 'i am sleepy!'
ciphertext1, authtag1 = encrypt(plaintext1)
ciphertext2, authtag2 = encrypt(plaintext2)

# "Decrypt" ciphertext1 knowing only the contents of plaintext2 and
# ciphertext2.
puts xor_strings(ciphertext1, xor_strings(plaintext2, ciphertext2))